Our objective is to understand the control of myosin expression in normal limb development and in a model system of hereditary pituitary dwarfism in mice in which myosin expression is abnormal. Up to seven myosin heavy chain isoforms are found in striated muscle in mammals, and they reflect the functional, developmental and pathological state of muscle tissue. Specific methodologies have been developed for the study of myosin isoforms, including biochemical, immunological and molecular cloning approaches. These will be applied to the study of the dwarf mouse model system in combination with an analysis of the endocrinological and neural control of myosin isoform expression. A comprehensive approach such as this is needed for full understanding of the relationship between molecular and physiological aspects of limb muscle development. The influence of hormonal factors on muscle development and regeneration may have a potential role therapeutic treatments of neuromuscular disease. Pituitary dwarfism has been reported to have palliative effects on such diseases, and therefore it is important to understand the endocrinological effects on muscle physiology. The effectiveness of thyroid hormones and analogues in promoting myosin isoform transitions will be studied, along with their effect on experimentally-induced regenerating muscle in rats and mice. In addition to biochemical analysis of myosin, immunochemical techniques will be used, employing available specific antibody reagents. Myosin transitions will be analyzed at the cellular level by immunocytochemistry. The isolation of large segments of the adult skeletal myosin gene will give information on potential regulatory regions in the DNA. The cloned gene will also provide probes to be used for specific analysis of myosin gene expression in dwarf mice and in rats. This will include measurements of mRNA levels, in situ hybridization of cloned DNA to localize mRNAs to specific muscle cell types, DNAse I sensitivity of the adult gene, and nuclear transcription experiments to measure directly the rate of gene expression in different experimental circumstances.